First Generation Antihistamines

Antihistamines are commonly used medications for the treatment of allergic reactions, insomnia, and nausea.  Antihistamines block histamine receptors, which reduces the effects of histamine in tissues.  Many of the drugs in this class also interact with other receptors including cholinergic, adrenergic, or serotonergic receptors, which results in variations of medication effects including adverse effects.  

Class Drugs

• Brompheniramine

• Chlorpheniramine

• Clemastine

• Cyclizine

• Cyproheptadine

• Diphenhydramine

• Dimenhydrinate

• Doxylamine

• Hydroxyzine

• Meclizine

• Olopatadine

• Promethazine

Class Mechanism of Action

Blocks histamine receptors

Histamine receptor blockade results in:

• Decreased nasal and bronchial secretions

• Relaxation of bronchioles

• Relaxes intestinal smooth muscle decreases diarrhea and cramping

• Decreased pruritus

• Increases peripheral resistance in blood vessels

• Decreases heart rate

• Vasoconstriction of capillaries, reducing fluid leakage

• Decreases gastric acid secretions

Many agents have other effects depending on their individual interaction with cholinergic, adrenergic, and/or serotonergic receptors

Class Applications

Allergies

Sleep aid

Motion sickness and nausea

Class Adverse Effects

Sedation (blocks central H1 receptors)

Dry mouth

Brompheniramine & Chlorpheniramine: Caution with concomitant use of MAOIs as both antihistamines are also 5HT reuptake inhibitors, so serotonergic effects can become toxic

Overdose

• Hallucinations

• Ataxia

• Convulsions

• Cardiovascular collapse in severe cases

Drug Table

Drug Name	Application	Comments	Image
Brompheniramine			"Broom" "Fence" "Ear" "Mine"
Chlorpheniramine			"Clorox" "Fence" "Ear" "Mine"
Clemastine			"Clemson" "Steam"
Cyclizine			"Cycle" "Liz"
Cyproheptadine	Anorgasmia in SSRI use; Appetite stimulation		"Cyclops" "Pro" "Heptagon" "Diner"
Diphenhydramine			"Dice" "Fence" "Hydrant" "Mine"
Dimenhydrinate			"Dice" "Men" "Hydrant"
Doxylamine			"Doxen" "Mine"
Hydroxyzine	AnxietyPruritus		"Hydro Flask" "Rind"
Meclizine	Vertigo		"Mecca" "Liz" "Rind"
Olopatadine	Allergic Conjunctivitis		"Old" "Pat" "Dining"
Promethazine	Nausea	Adverse Effect; Tissue necrosis with subcutaneous administration or extravasation from IV administration	"Pro" "Meth" "Zine"